The present invention relates generally to centrifugal compressors, and in particular, to a diffuser for a centrifugal compressor.
Centrifugal compressors have a wide variety of industrial and aeronautical applications, including gas turbine engines, fluid pumps and air compressors. Centrifugal compressors generally consist of at least two main components: an impeller and a diffuser.
Pipe diffusers, generally having circumferentially spaced frustro-conical discrete passages, are commonly used to perform these functions. Typically, the radially extending passages are angled from the radial direction such that their center lines are all tangent to a single tangency circle. A partially vaneless space is therefore created where the passages intersect, between the tangency circle and an outer leading edge circle. The intersection of circular pipe diffuser passages creates symmetrically located elliptical leading edge ridges formed on the leading edge circle. When such a diffuser is placed around an impeller, the exit flow from the impeller will enter the diffuser at the tangency circle, flow through the partially vaneless space, and enter the discrete passages of the diffuser.
One cause of centrifugal compressor pressure losses, which negatively affect the compressor efficiency and therefore the overall compressor aerodynamic performance, is any mismatch between the impeller exit flow angles and the inlet angles of the diffuser. As the distribution of the impeller fluid exit angles from the impeller hub to the shroud end of the impeller vanes is not uniform, it follows that ideally the leading edges of the diffuser passages would be shaped to provide a corresponding profile of inlet angles. Traditionally used diffuser pipes having a circular cross-section form generally oval diffuser passage leading edges, which fail to provide such an ideal match with the impeller fluid exit angles.
It is an object of the present invention to provide a diffuser capable of improving compressor efficiency.
It is a further object of the present invention to provide an improved incidence match between the impeller exit air angles and the diffuser leading edge angles.
Therefore, in accordance with the present invention, there is provided a centrifugal compressor including an impeller and a diffuser, the impeller having an inner integral hub with vanes thereon, being adapted to rotate within an outer shroud about a central longitudinal axis, and having a defined hub-to-shroud distribution of fluid exit angles, the diffuser, being downstream from the impeller, comprising: a plurality of circumferentially spaced discrete passages at least partially defining fluid paths through the diffuser, and being angled such that adjacent discrete passages intersect each other to form an annular semi-vaneless diffuser inlet space; the discrete passages downstream of the semi-vaneless space each having an inlet therefrom and an outlet with a greater cross-sectional area than the inlet; intersection of the annular semi-vaneless space and each discrete passage defining a leading edge thereof; each discrete passage being defined by a wall bounding a cross-sectional area, the wall comprising at least a first substantially rectilinear portion and a second opposed convexly curved portion; the first substantially rectilinear portion being adjacent the hub of the impeller and the second opposed convexly curved portion being adjacent the outer shroud; and the leading edge of each discrete diffuser passage providing a close incidence angle match with the fluid exit angles of the impeller. The walls defining the discrete passages downstream of the semi-vaneless diffuser portion are removably engaged with a diffuser case.
There is also provided, in accordance with the present invention, a diffuser for use with an upstream impeller in a centrifugal compressor, comprising: a plurality of circumferentially spaced discrete passages defined by walls bounding cross-sectional areas, the walls at the inlets of the passages comprising at least a first substantially rectilinear portion and a second opposed convexly curved portion; adjacent discrete passages intersecting each other at their respective inlets to form an annular semi-vaneless space at an inlet of the diffuser; intersection of the annular semi-vaneless space and the discrete passages defining swept back leading edges thereof, providing a close incidence angle match with a hub-to-shroud distribution of fluid exit angles from the impeller. The walls defining the discrete passages downstream of the semi-vaneless difuser portion are removably engaged with a compressor case.